Do you know any subreddit about this specific topic?

I am fairly new using Reddit but I wanted to give it a try. I am looking for a subreddit where I can offer a service for free in exchange for a review or case study about my work with them (like here is the Beta testing subforum or the review copies on the marketplace.

Do you know any subreddit about it or a way to find it? Because I have tried posting it on normal subreddits about my niche (let's say SEO) with the [GIVEAWAY] prefix and in most all the subreddits it was taken down.

not sure this will be of much help but worth looking at r/ProductTesting/

Check the list of all https://www.reddit.com/r/ListOfSubreddits/wiki/listofsubreddits#wiki_usa to see what do you want, hope it helps

Don't make it too obvious with a [GIVEAWAY], of course they will take that down.
Say you're a student or look to build your portfolio.
r/Entrepreneur, r/forhire, r/smallbusiness work for that.